Nowadays, a measurement of the intensity of the optical field in integrated optical waveguide devices with a photon scanning tunneling microscope (PSTM) is reasonably routine. However, if one could visualize the evolution of the phase of the light field in real-space, it would yield new and detailed information concerning the properties of wavelength multiplexers, mode converters, etc., of which the operation depends critically on the phase of the light. We believe we present the first phase maps of light in integrated optical waveguides obtained with a heterodyne interference PSTM.